Data model docs: Add toplevel header

This was added automatically when this document was in the wiki, but it
needs to be inserted manually now that it is in the main Git repo.
Data model docs: Unify header capitalization
Data model docs: Remove the "Scope Details" section

This was an obsolete proposal. Also, this kind of stuff will go into the
detailed format documentation.
Data model docs: Add a "Root" header

The discussion below the added header is not "Basics" anymore, it's on
the same level as "Representing Scopes".
Merge pull request #73 from SUSE/data-model-docs-improvements

[Review] Request from 'dmajda' @ 'SUSE/machinery/data-model-docs-improvements'